This type of process/device may typically be incorporated in a more general process for manufacturing nuclear fuels using sol-gel techniques (i.e. techniques exploiting precipitation reactions). This type of process makes it possible to limit dispersion of potentially radioactive/contaminant substances during the production of spherules/spheroids of fuel materials, these spheroids, of controlled size and shape, themselves subsequently being (potentially) incorporated into fuel elements potentially manufactured using deposition techniques (fuel of the TRISO/VHTR type, for example), the acronym VHTR in expressions such as “VHTR cycle” standing for “very high-temperature reactor” and the acronym TRISO in the expression “TRISO fuel” standing for tristructural-isotropic, TRISO fuel being a specific type of fuel micro-particle. The latter particle has a spherule of fissile material composed of UOx (sometimes UC or UCO) at its center, this central spherule being coated with four layers of three isotropic materials. The four layers are a porous buffer layer made of carbon, an internal dense layer of pyrolytic carbon (PyC), a ceramic layer of SiC used to keep the fission products at high temperatures and to reinforce the structural integrity of the TRISO particles, and an external dense PyC layer. It is also possible and preferable to use vibrocompaction techniques to produce said spheroids, the range of particle sizes targeted in the latter case being wider and potentially more difficult to achieve.
Thus, droplets of fuels dissolved in a liquid phase that are precipitated using a conventional GSP (gel supported precipitation) sol-gel process have been developed, these droplets having properties and production conditions suitable (in terms of sphericity, droplet size, generation rate, etc.) for the subsequent synthesis of solid spheroids that can be used in more global processes for producing nuclear fuels such as illustrated in FIG. 1 (mainly with application to vibrocompaction in comparison to V/HTR applications, which tolerate relatively large spherule diameters, i.e. diameters larger than 300 μm) which illustrate all of the following steps:
step 1: a solution containing the dissolved fuel is prepared;
step 2: droplets of controlled size and shape are generated;
step 3: a gel supported sol-gel process is used;
step 4: solid spheroids are produced;
step 5: said spheroids are deposited, or in step 5a: vibrocompaction is used to fill a fuel cladding; and
step 6: V/HTR fuel elements are produced, or in step 6a: fuel obtained by vibrocompaction is produced.
In the prior art, droplets are generated by a number of devices/processes. Nevertheless, in the context of the aforementioned situation (generation of spheroids for the manufacture of nuclear fuel) a certain number of specific constraints/objectives mean that certain prior-art techniques must be selected. It will be noted that the sol-gel precipitation process is an effective method of limiting the risk of contaminant dispersal since it does not employ radioactive powders. In contrast, in this technology the risk of blockage and the size and shape of the solid spheroids, precipitated in order subsequently to be used in a more global manufacturing process, must be controlled as is illustrated in FIG. 2, which schematically shows the deformation that droplets Gou experience when they strike a given liquid surface S (typically the precipitation solution) or, more precisely, the variation over time of the diameter ratios Dmax/Dmin in two directions, respectively parallel and perpendicular to said struck surface, and showing a variation Δ in sphericity.
Thus, the droplets must themselves have:                controlled sizes and shapes (typically in a range that may range from a few tens to a few hundred microns and with a sphericity ratio (Dmax/Dmin) lower than 1.1); and        the droplet generation rate must ideally be low (in order to limit deformation of the droplets (characterized by the aforementioned sphericity ratio) when they strike the liquid containing the element enabling precipitation in the context of the sol-gel process).        
Generally, the main technologies discernible in the prior art for generating droplets are the following:
—Nebulization Using a Jet of Gas:
This technique is quite common and is exploited in various industries. It employs, to cause a liquid to fragment, kinetic energy delivered by the relative movement (relative to the liquid to be fragmented) of a gas generating a high shear force and consequently the subsequent desired fragmentation. Examples of this technology are notably described in patent applications US 2010/0078499 and EP 1 888 250. The main drawback of this type of technology is that it employs jets of gas, which are disadvantageous in the nuclear industry (because they make contact with a contaminant they are a source of gaseous waste requiring filtering). Moreover, these types of droplet generator frequently become blocked due to the need for the liquid feed outlet to have a limited cross section so as not to increase the flow rate of the gas used to nebulize the liquid. This limitation results in a high risk of blockage, it being very disadvantageous in the nuclear industry to have to carry out repair/maintenance work on devices used to process radioactive material (respecting the ALARA principle: the ALARA principle is one of the basic principles of ionizing radiation protection. The objective being to reduce the individual and collective dose received by the personnel of nuclear service providers).
—Generation of Droplets Using a Vibrating Injector:
This technology is widely used in sol-gel processing to generate calibrated droplets. In this type of technology, a liquid jet flows under gravity through a calibrated orifice subjected to vibration at a given frequency thus fragmenting the jet into monodisperse droplets. In devices with vibrating orifices (example described in patent application WO 2006/048523) the jets are almost cylindrical and the diameter of the droplets generated is about twice that of the orifice. This greatly limits the usefulness of this technology and results in a high risk of blockage that only increases as the diameter of the desired droplets decreases. It will also be noted that in order to be able to change notably the diameter of the droplets generated by this type of generator, it is necessary to change the diameter of the nozzle through which the jet is emitted, which is moreover not a degree of freedom often required by industry and contradicts the aforementioned ALARA principle. In addition, this type of injector is limited to generating small droplets (smaller than 100 μm in size) for high viscosity (typically higher than 50 cp) liquids, as illustrated in FIGS. 3 and 4, which respectively show the variation in the head loss in a straight outlet injector as a function of the diameter Dinj of the injector, and the variation in the time before a straight outlet injector becomes blocked Tav/Bou for ejection of a viscous liquid such as, for example, an aqueous solution rich in polyvinyl acetate (PVA), the parameter Dmin corresponding to the minimum diameter before the head loss becomes too great.
—Generation of Droplets by Fragmenting a Liquid Jet Using a Mechanical Rotary Effect:
This type of device allows a jet to be fragmented via the mechanical shear force induced by a moving (most often in rotation) stop that makes contact (at high speed) with the liquid jet. This type of device, also called a jet-on-surface impinging atomizer, is accompanied by a substantial loss of material (low yield) which may be disadvantageous for industrial production, the droplets thus produced also having emission speeds close to the speed of the element that cuts the jet (or the speed of the incident liquid), thereby failing to achieve the aforementioned objective.
—Generation of Droplets Using Ultrasound:
In this type of technology, the free surface of a liquid to be fragmented is excited by a source of acoustic waves. Columns of liquid appear at the surface of the liquid, from which columns very small droplets escape with a quite wide droplet size dispersion, this type of generator moreover not allowing droplets having a diameter greater than a few tens of microns to be easily produced. Moreover, since the droplets are near the liquid source, it is not easy to carry out a step of gelling the droplets thus formed without running the risk of precipitating the liquid source itself. Moreover, the fragmentation is constrained by the natural resonant frequency of the free surface of the liquid. The degree of freedom for adjusting the droplet diameter thus achievable is almost nonexistent, which is a substantial limitation from a processing point of view.
—Generation by Atomization (Rotary Optionally Vibrating Device):
This type of device is based on the use of a centrifugal force to produce a liquid film on the surface of a rotating member. At the periphery of this member (often a disk or a wheel) droplets are generated; whether the droplets form and their size depend on the parameters of the rotation (notably the rotation speed), the surface finish of the rotary element, and the physico-chemical properties of the liquid to be atomized. The fluids that the solution of the present invention proposes to fragment are liable to lead to blockage-type effects; it is not viable to base the fragmentation (as described in patent application WO 2005/102537) on the surface finish and characteristic size of elements (often called teeth) that have a geometry that is necessarily subject to change due to clogging-related effects. Moreover, the emission velocity of the droplets is intrinsically high (of the same order as the linear speed of rotation of the rotating plate) making it difficult to obtain spheroids by gelling the droplets via impact with a precipitation solution. Furthermore, rotary atomizers are not very suitable for fragmenting viscous liquids since they require communication of a high rotation speed; moreover, stability of the fragmentation then causes control-related problems.
—Generation of Droplets Using Impinging Jets:
This type of device, which is notably described in patent application WO 2009/047284, generates droplets by making impinging jets strike one another. This type of generator leads to, if it is not used under specific conditions such as those for which it was developed (very hot medium such as a flame or a plasma cone), a disadvantageous loss of liquid. Moreover, the emission velocity of the droplets is of the same order as the strike velocity of the liquid jets, again failing to achieve the aforementioned objectives/meet the aforementioned constraints.
Thus, of all of the techniques known in the art, no technology for generating droplets allows all of the following criteria to be met:                generation of droplets via fragmentation of potentially (highly) viscous liquids;        generation of droplets of liquid capable of being subjected to precipitation or other effects leading to a high risk of blockages;        generation of droplets of liquid capable of being emitted at low droplet speeds (possibly as low as 0.1 m/s or even less); and        generation of droplets with a wide and adjustable droplet size distribution (from a few tens of microns to a few hundred microns).        
It will be noted that certain of these objectives and/or constraint functions are antinomic/contradictory, especially as regards the following desired outcomes:                fragmentation of viscous liquids without emission of droplets at high speeds. Specifically, high viscous forces conventionally require high kinetic energies or shear forces to be used which consequently induce sprays/the generation of high-speed droplets; and        generation of droplets of small size without using elements sensitive to blockage effects. Specifically, in most droplet generators the droplets are obtained using mechanical elements the size of which is about the size desired for the droplets to be generated. This size is either the diameter of the actual injector when the technology employed is based on this type of element (the size of the droplet then being about twice the diameter of the outlet), or the size of profile elements (teeth, needles, etc.) allowing instabilities to be generated or shear forces to be applied inducing the fragmentation of elementary volumes the size of which is similar to that of these elements.        